It comes back after the bullet exits the barrel. The gas behind the bullet goes down the tube and unlocks the lugs while pushing back the bolt (I think). So your second idea was right. The gas behind the bullet.

"Necessity is the plea of every infringement of human freedom. It is the argument of tyrants, it is the creed of slaves." -William Pitt, 1

Yes it is the pressure behind the bullet for a micro second that actuates the piston/bolt on gas operated semi auto. This was invented and perfected by the master himself John Moses Browning. Most other technological advancements of modern firearms can either be directly or indirectly attributed to him as well.

With one possible exception, it's the gas behind the projectile.. The "Bang" system supposedly began unlocking with the pressure preceeding the projectile..(I have no hard evidence of this, having read only ONE description of it doing this.)

It kind of depends on the location of the gas port as well.. The M-1 Carbine used a short stroke, high pressure gas system, tapping the gas off closer to the chamber than many..

With the AR-15, it uses the bolt, and bolt carrier to serve as the gas "piston" in it's system..(This is a VERY simplified description..)

Another form of gas operation is seen in some handguns..The Steyr GB is one I'm familiar with that uses this, tapping gas to hold the slide shut, til the pressure drops to allow it to act as a blowback..

If you can find it, "Automatic Weapons" by Johnson, and Haven is a great, but out of print book on how many automatic weapons function, and evolved..

2) The powder burns and the gas generated increases in pressure, sending the bullet down the barrel.

3) The bullet passes by the gas port in the barrel, exposing the port to the high pressure gas behind the bullet.

4) Gas is conveyed back to a piston by a tube of some length, depending on the specific weapon. The gas pressure, now decreasing because the gas is expanding into an increasingly large volume, strikes the face of the piston, forcing it to move.

5) The piston is in most cases (M1 Carbine is an exception) connected to the operating mechanism which will tilt (FAL, SKS) or rotate (AK, AR, M1 Garand, M14, etc.) the bolt to unlock it. In the M16/AR15, gas enters the carrier and exerts force against the firing pin and the carrier. This forces the carrier to the rear, rotating and unlocking the bolt from the barrel. There's a bit more to it, but this is the gist of it.

6) Due to the mass of the operating rod, bolt carrier, bolt, etc, and the inherent friction and other factors, the bullet will have exited the barrel before the operating mechanism actually moves much, if at all. The gas actually provides an "impulse" against the piston in the form of a pressure spike that rapidly builds and the drops to atmospheric pressure as the bullet exits the barrel. In this manner, the movement of the operating system after the gas impulse does not affect the aim and thus accuracy of the weapon.

7) There is some appreciable force of the case walls against the chamber and the boltface upon firing. The gas pressure pushing against the case walls and head tends to instantaneously expand and "lock" against the chamber walls, limiting the amount of force on the bolt face. In gas-operated weapons the gas pressure has to drop to atmospheric for the case to "relax" and allow the extractor to pull it out of the chamber.

BLOWBACK OPERATION:

In straight blowback weapons like most pistols chambered for 9x18, 9x17, .32 ACP, .25 ACP, .22LR, etc, the cartridge case itself is the piston against which the gas pressure pushes, countered by the mass of the slide and the force of the recoil spring. You don't see very many straight blowback pistols in 9mm or larger (the Highpoint line of 9mm and 45 ACP caliber heavy fishing sinkers and small watercraft boar anchors comes to mind) because the mass of the slide and the force of the spring has to be proportionately larger, so much so that it would take a Schwarzenegger to pull back the slide. The answer to this problem is DELAYED Blowback, in which the barrel is locked to the slide for the first few millimeters of movement, then cammed down out of the way to allow the slide to fully travel to the rear(ex. 1911, BHP, Tokarev TT, SIG, Ruger P-series, CZ-75, etc.). This brief period of time when the barrel is locked to the slide after cartridge ignition allows the whole works to get started moving rearward while the gas pressure is still quite high. The pressure drops nearly to atmospheric at about the time the barrel unlocks from the slide and the slide continues rearward to eject the spent case and chamber a new round from the mag. The same principle holds for rotating bolt pistols (ex. Colt 2000, remember that one? I don't either.) and roller-locked (ex. CZ-52) and tilting-cam locked (ex. Walther P-38 and Beretta 951, 92) handguns.

Yes the instant of unlocking is determined by the placement of the gas port on the barrel and the inertia of the locking mechanism. Gas operated autoloading shotguns work the same way. By the time the bullet or shot/wad has moved forward and uncovers the gas port they are moving practically at the muzzle velocity because the ports are near the muzzle. It takes some time for the gas to accelerate the piston (shotgun) or actuate the bolt carrier (AR15). Either way by the time the bolt opens the projectiles are already out the muzzle and the pressure in the barrel has dropped to a safe level.

If I understand the AR mechanism correctly the bolt initially is locked and cannot move back so the carrier initially moves backward. As the carrier moves back the cam pin rotates the bolt, until the bolt is unlocked. The bolt is then pulled back by the inertia of the carrier?

Actually as I recall it was a big plate infront of the barrel with a hole the size of the caliber of the rifle. When you fired it, the gas leaving the barrel caused the plate to move forward which then moved the lever action to load another round. The first semi auto rifle. Somewhere I have seen either a drawing or a picture somewhere, I just can't remember where.

Originally Posted By Orion526:Thats interesting. Who would have ever thought of that idea and applying it to a rifle.

Bet they are rich though.....

They're acually dead.

i belive John M. Browning invented gas operation, first in a shotgun, then a rifle.

Correct but the story I heard was that he was experimenting with a lever action rifle, using a hole in the barrel (gas port) and a piston attached to a rod to actuate the lever.

most likley. i belive he first got the idea from seeing gas from a shotgun move stuff (grass or weeds of something) out in front of the gun. i had never heard the lever piston thing, but i dont doubt it. im pretty sure the first gun sold was a shotgun (think A-5) and the first rifle was similar (looking anyway) to remingtons autoloading guns.

Around the mid-1890s Colt introduced their Machine Gun invented by J M Browning. It was called the "potato digger" because a hinged lever on the bottom of the barrel swung through an arc about 150 degrees or so to actuate the bolt and feed mechanism. A cup on the end of the lever surrounded a nipple through which gas was tapped from the barrel. It was chambered for the .30-40 Krag cartridge, if I recall.

J M Browning did invent a gas-operated lever action by using a "gas trap" at the muzzle, for all intents and purposes working like the flap on a tea kettle spout. It wasn't commercial, but proved the concept and got J M working on the Colt machine gun design and later BAR.

you wouldnt need a gas rod to operate a leveraction. actually a lever action rifle is one of the easyest guns to convert to full auto. if ya got one fire it without holding the lever up you'll se what i mean....